A serious problem exists in controlling the feeding and weighing of individual weighments of commodities at practical production speeds while at the same time assuring that legal requirements are met without `giving away` product. Variations in individual weighments are unavoidable. These variations are caused by factors such as changes in the amount of product reaching the scale after the cut-off signal is given to the feeding device, variations in product density, the hopper head load and the thickness of the product on the feeder troughs.
The laws of most states which deal with allowable package weights of consumer products are patterned after the "Model State Packaging and Labeling Regulation" as adopted by the National Conference on Weights and Measures which is sponsored by the National Bureau of Standards of the U.S. Department of Commerce. The 1976 edition of this regulation states, in Paragraph 12.1.1, that the variations from the declared net weight to be allowed in individual packages of consumer commodities "shall not be permitted to such extent that the average of the quantities in the packages of a particular commodity, or a lot of the commodity that is kept, offered, or exposed for sale, or sold, is below the quantity stated, and no unreasonable shortage in any package shall be permitted, even though overages in other packages in the same shipment, delivery or lot compensate for such shortage." The regulation also provides that "[t]he magnitude of variations permitted under Sections 12., 12.1., 12.1.1., and 12.1.2., of this regulation shall be those contained in the procedures and tables of the National Bureau of Standards Handbook 67, `Checking Prepackaged Commodities`."
The average net weight of a group of packages is the quotient obtained by dividing the sum total of a group of net weights by the number of net weights. A weight control system providing optimum performance would maintain the average weight of all packages at the declared weight, but not below, and all "unreasonable" underweight charges would be corrected rather than rejected. The weight control system would thus be required to hold two variables, viz., the average weight and the underweight limit. There is interaction between these two variables. In this regard, if a control system should hold a particular average weight at the time the initial feed and weighing cycle is completed, this average will obviously be changed if any "unreasonable" shortages are either corrected or rejected. Stated simply, if the system provides for correcting underweight charges by adding product, the charges to which product has been added will clearly affect the overall average.
One can readily appreciate the considerable advantages of a weight control system that will assure the user of the system and the consumer of the product of absolute compliance because of the legal and goodwill implications. In this regard, a system which, by its inherent design, holds an average representing the weight of every package produced during the operation of the system would provide convincing evidence of an intent to comply with the law. On the other hand, one will also appreciate the very important advantages of a system which provides this kind of compliance without product "giveaway" in view of the very great economic cost of giveaway when many millions of packages are involved.
Other factors should also be considered. Thus, if the average net weight is to be maintained equal to or slightly above the declared quantity, it is necessary to provide a control system that is extremely accurate and responsive to very small errors. Further, if the net weight of a package is to be accurately controlled, the weight of the charge should be checked before the charge is placed in the package in that if the charge is weighed in the package the charge weight will be affected by uncontrolled variations in the package tare weight.
Conventional weight control systems that claim to control the average weight periodically check a sample of weighments, determine the amount of error in the sample and cause a correction in the weight set point to be made. The intent of such systems is to make a correction that will produce correct weights in the future. However, the correction does not necessarily make up for the extra product or the missing product that was in the packages prior to making the correction. Such "sampling" systems are thus based on the expectation that the errors will average out. However, there is no guarantee that this will happen and no indication that the desired average weight is being maintained. Moreover, as mentioned above, if the control system considers the initial weight of the charge and the charge is subsequently rejected or product is added to the charge, this change in the charge weight must be considered because the subsequent events referred to will necessarily alter the average. Thus, the control system must separately consider the initial weighment and the weight after correction. To explain, if only the final weighment is considered, the control system could, for example, be producing charges which are not up to weight and which are, therefore, subsequently corrected to end up "heavy". Under this circumstance, the control system would then erroneously provide for less product (in view of the heavy charges at final weight) when actually more product is needed (in view of the fact that the packages being produced are light prior to the addition of product in the correcting step).
As will appear from the discussions hereinbelow, one aspect of the present invention concerns summing, algebraically, the deviations from the desired average to be maintained. There are prior art systems which, in effect, provide algebraic summation of a deviation from a desired weight over a predetermined sampling period. An example of such a system is that disclosed in U.S. Pat. No. 2,697,580 (Howard). However, such systems clearly suffer the disadvantages of the sampling-type systems discussed hereinabove, i.e., the systems provide corrections based on the events taking place during each sampling period and actually ignore the previous history of the operation prior to the particular sample in question. Stated differently, with such systems the results of each individual sample is, in essence, thrown out or discarded after the indicated action has been taken so that the next sample is not based on the first sample but rather is exclusively concerned with the events taking place during its sampling period. Thus, as stated, there is simply no assurance with systems of this type that the desired overall average is being maintained.
Other patents of possible interest relative to the present invention include U.S. Pat. Nos. 2,076,617 (Cleaves), 2,538,346 (Wood), 2,354,087 (Taymer), 2,037,484 (Raymer et al), 3,274,377 (Morison), 3,724,569 (Blodgett), 3,643,752 (Blodgett), 2,688,459 (Merrill et al), 2,628,055 (Knobel et al), and 3,862,666 (Muskat et al) although it is not represented that this listing is complete or that closer prior art does not exist.